In recent years, an SR motor which is compact and has a high output torque has been used as, for example, an actuator for various systems mounted on a vehicle in many cases. One example in which the SR motor is used in a shift range switching apparatus (shift by wire: SBW) is described below.
An automatic transmission of the vehicle has the shift range switching apparatus (including a parking switching mechanism). Although a driver manually shifted gear ranges formerly, the shift range switching apparatus in which an SBW actuator having the SR motor switches a shift range switching unit has been recently spreading in the market.
(Problem 1)
The output torque of the SR motor is expressed by the following equation.T=0.5I2×dL/dθ
wherein T is the output torque, I is a current, L is an inductance of a coil, and θ is a rotation angle.
As described above, the output torque (T) of the SR motor varies depending on the current (T) flowing though the coil and the changing rate of inductance (dL/dθ).
When a large current flows though the coil, as shown by a dotted line A′ in FIG. 2B (θ=0° represents an angle at which a stator tooth and rotor tooth begin to face each other), the changing rate of inductance (dL/dθ) lowers due to magnetic saturation as a facing area between the stator tooth and rotor tooth increases. At this time, because the current is large, the fluctuation of the output torque becomes large. That is, as shown by a dotted line B′ in FIG. 4, the output torque of the SR motor sharply falls off as the facing area between the stator tooth and rotor tooth increases. As described above, when the large current is given to the coil, there is a problem that large torque fluctuation (torque ripple) occurs.
(Problem 2)
Conversely, when a small current is given to the coil, as shown by a dotted line C′ in FIG. 3B, the changing rate of inductance (dL/dθ) lowers as the facing area between the stator tooth and rotor tooth increases. However, because the current is small, the fluctuation of the output torque becomes small. That is, as shown by a dotted line D′ in FIG. 4, the output torque of the SR motor fluctuates within a narrow range in the whole region of a rotor rotation angle.
However, even though the current is small, as shown by the dotted line D′ in FIG. 4, the output torque falls off as the facing area between the stator tooth and rotor tooth increases, as the result that the changing rate of inductance (dL/dθ) lowers as the facing area increases. That is, when the small current is given to the coil, the fluctuation of the output torque can be suppressed, but the output torque falls off as the facing area increases.
(Problem 3)
The current flowing through the coil of the SR motor varies in response to a “battery condition” or “temperature”. When the temperature is high, the output torque becomes low since the electrical resistance of the coil becomes high. Also, when the battery voltage is low, the output torque becomes low since the current of the coil decreases. For these reasons, when the temperature is high and the battery voltage is low, the current of the SR motor becomes minimum (high temperature×low voltage=minimum current).
The SR motor installed in the shift range switching apparatus is required to generate a necessary torque under the worst condition of the “high temperature×low voltage=minimum current”. However, if the necessary torque is ensured under the worst condition of the “high temperature×low voltage=minimum current”, the SR motor generates an excessive output torque under the opposite (best) condition of “low temperature×high voltage=maximum current”. As a result, there is a possibility that loads (objects driven by the SR motor including a rotation transfer system between the SR motor and the driven objects) are mechanically damaged.
For example, as shown by the dotted line B′ in FIG. 4, there is the possibility of the loads mechanically damaged as the result of the excessive output torque generated near the angle at which the stator tooth and rotor tooth begin to face each other at the time of the maximum current.
In order to suppress the torque ripple and to decrease the maximum torque, Japanese Patent Publication No. 2002-10595 discloses a switched reluctance motor as shown in FIGS. 11A, 11B, in which an uneven section is formed at the tip of the rotor tooth 24a (24b) (or stator tooth 23a (23b)) and concavities and convexities of the uneven section in a pair of rotor teeth 24a, 24b (forming a magnetic loop) are arranged in opposite phase.
However, the above-described switched reluctance motor can not sufficiently solve the problems that the large torque fluctuation occurs and the excessive output torque is generated when the large current (particularly, maximum current) flows though the coil.